Method for production of alkali metal chromates

ABSTRACT

This invention relates to a method for producing chromates, especially for producing alkali metal chromates. The method comprises following steps: the obtaining of a mixture of alkali metal hydroxide, alkali metal chromate, and ferrous residue after the reaction of chromite ore with an oxidant in the reactor in the presence of molten salt or in aqueous solution of alkali metal hydroxide, the obtaining of a leaching slurry by leaching the reaction products with aqueous solution of alkali metal hydroxide, the separating of the primary chromate product from the leaching slurry, the obtaining of pure chromate crystal by purifying the primary chromate product. Both the primary chromate product and the pure chromate crystal can be used as the raw materials to manufacture other chromium compounds. Compared with the currently-used roasting method, the method has the advantages of decreasing the reaction temperature by about 700° C., of improving the chromium recovery rate up to nearly 100%, of minimizing the amount of the ferrous residue by not adding calcium-containing auxiliaries, and of completely eliminating the disposal problem of the chromium-containing residue and the environmental pollution due to the addition of calcium-containing auxiliaries, by using ferrous residue as the raw materials for other industries after extraction of magnesium.

FIELD OF THE INVENTION

This invention relates to a method for producing chromates, especiallyfor producing alkali metal chromates.

BACKGROUND OF THE INVENTION

Chromates are chemicals of vital importance. As the raw materials formanufacturing a series of chromium-containing chemicals such asdichromate, chromic oxide and chromium anhydride, chromates are widelyused in leather tanning, pigment manufacturing, dyestuff manufacturing,metallurgical industry, materials industry, metal plating, porcelainenamel production, wood preservation and pharmacy.

Manufacturing chromates from chromite ore is an important basicmaterials industry crossing the chemical and metallurgical industries.The current industrial production method is mainly the roasting ofalkali metal carbonates. It is a high-temperature oxidative roastingreaction of chromite ore, alkali metal carbonates, and air in a rotarykiln. The reaction temperature is in the range from 900 to 1200° C. Itneeds the adding of alkali additives such as limestone, dolomite, andferric oxide in the reaction process. Primary chromate products areobtained by leaching the roasting products with water. Pure chromateproducts or other chromium-containing products can be obtained afterimpurity removal and evaporation crystallization. However, the roastingprocess produces a great amount of highly toxic chromium-containingwaste residues, creates serious environmental pollution, and the methodremains a global unresolved environmental problem so far.

Presently, the traditional soda-ash roasting technology is still in usein many countries including China. Calcium-containing auxiliaries,mostly alkali compounds, are added in the process of roasting. Thefunction of the calcium-containing auxiliaries is to reduce the masstransfer resistance in the liquid film of chromite ore during theroasting process, consequently strengthening the mineral decompositionprocess, reducing the consumption of alkali as raw materials, improvingthe product quality, and eliminating the unfavorable effects onroasting, separation and product quality of the acidic components suchas aluminum oxide and silicon oxide in the chromite ore. In thisprocess, the amount of calcium-containing auxiliaries is twice as muchas the amount of chromite ore. The reaction temperature is as high as1200° C. However, the recovery rate of chromium is below 80 percent and2.5 to 3.0 tonnes of highly toxic chromium-containing residues areproduced with the production of 1.0 tonne of chromate products. Thecurrently-used disposal method of the chromium-containing residues ismainly landfilling, which not only wastes the chromium resources andcalcium-containing auxiliaries but also results in great environmentalthreats potentially.

To minimize the discharging amount of chromium-containing residue andmaximize the recovery rate of chromium, investigations on less-calciumroasting and calcium-free roasting processes were carried out in manycountries. The patent technology of calcium-free roasting, which hasbeen industrialized in developed countries including UK and USA, andless-calcium roasting technology are both improved roasting technology.In this technology the recovery rate of chromium is increased bysubstituting a certain amount of chromium-containing residues forcalcium-containing auxiliaries and prolonging the staying time of thechromium-containing residue in the rotary kiln so as to further oxidizethe chromium in the chromium-containing residue. In the meanwhile, theamount of auxiliaries can be reduced, even to zero, and thereforedecreases the discharging amount of chromium-containing residues.

As the most advanced roasting technology presently, the calcium-freeroasting technology achieves higher chromium recovery rate and lesschromium-containing residue discharging amount than the traditionalsoda-ash roasting technology. But the total chromium recovery rate ofcalcium-free technology is only 90%, and 0.8 tonnes ofchromium-containing waste residues are discharged with the production of1.0 tonne of products, and the environmental pollution problem ofchromium-containing residues remains unresolved.

Manufacturing chromates via molten salt method has been reported since1980s. In this method, chromite ore is oxidized and decomposed by air inthe molten salt of alkali metal hydroxide or carbonate. The amount ofalkali metal hydroxide or carbonate is higher than their stoichiometricamount for decomposing chromite ore. The reaction temperature is 500 to900° C., higher than the melting point. This method does not involve thekey technology of continuous oxidation and phase separation in moltensalt media and therefore is not industrially applicable.

In a summary, all the methods available for producing chromates producesa great amount of chromium-containing waste residues and results inserious environmental pollution. Furthermore, none of these methodsachieves an approximately 100% of chromium recovery rate and thecomprehensive utilization of other valuable components in the chromiteore such as aluminum, magnesium and iron, therefore resulting inresource waste of chromite ore.

SUMMARY OF THE INVENTION

The invention is to cover such shortages in the current chromateproduction method as serious environmental pollution, low utilizationefficiency of chromite resources, and high reaction temperature, and toprovide an industrially operatable auxiliary-free production method forchromates.

It is the principles of the method for producing chromates provided bythe present invention that the multi-phase reaction of chromite ore withan oxidant is performed in the molten salt or in aqueous solution ofalkali metal hydroxide at a temperature range from 200 to 550° C. In thereaction, the trivalent chromium in the chromite ore is oxidized tohexavalent chromium in the form of alkali metal chromates. The oxidationreaction with oxygen as the oxidant proceeds according to the followingequation:14FeO.Cr₂O₃+56MeOH+8O₂=28Me₂CrO₄+7Fe₂O₃+28H₂O

Where Me denotes alkali metal, typically sodium or potassium.

The objectives of the present invention are achieved through thefollowing technical solutions:

The method for producing chromate provided by the invention includes:the chromite ore is decomposed with oxidant in the molten salt or inaqueous solution of alkali metal hydroxide, the leaching slurry isobtained via leaching of the reaction products with aqueous solution,and the primary chromate products are separated from the leachingslurry. In detail, the method includes following steps:

(1) Chromite ore decomposing. A mixture of alkali metal hydroxide,alkali metal chromate, and ferrous residue can be obtained after thereaction of chromite ore with oxidant in the reactor in the molten saltor in aqueous solution of alkali metal hydroxide. The weight of thealkali metal hydroxide used is two to eight times as much as that of thechromite ore used. The amount of the oxidant is either equal to or abovethe stoichiometric amount for decomposing chromite ore. The water amountin the reaction system is in the range from 0 to 50% by weight of thetotal amount of all reactants. The reaction temperature is in the rangefrom 200 to 500° C. The reaction time is in the range from 1 to 20hours.

(2) To obtain a kind of slurry with an alkali metal hydroxide content of30% to 70% by weight by leaching the mixture obtained in Step (1) withan aqueous solution of alkali metal hydroxide with a concentration rangefrom 0 to 30% by weight.

(3) To separate the leaching slurry obtained in Step (2) and obtainprimary alkali metal chromate product, ferrous residue, and alkaliliquor respectively.

The method for producing chromates provided by the invention alsoincludes the purification of the primary chromate products tomanufacture pure chromate crystal. It is carried out according to thefollowing steps:

(4) To dissolve the primary chromate products obtained in Step (3) inaqueous solution and obtain chromate aqueous solution.

(5) To adjust the pH value of the chromate aqueous solution obtained inStep (4) to the range from 3 to 9 with acidifying agent, filter out theprecipitates, and obtain pure chromate aqueous solution.

(6) To heat the pure chromate aqueous solution to evaporate the waterand obtain pure chromate crystal and mother liquor after crystallizing,filtering, and drying.

The said oxidant in Step (1) includes air, oxygen, sodium nitrate,potassium nitrite, sodium peroxide, potassium peroxide, or mixturesthereof.

The said alkali metal hydroxide in Step (1) includes the alkali metalhydroxide recycled from Step (3), the alkali metal hydroxidesupplemented, or mixtures thereof.

The said alkali metal hydroxide in Steps (1) and (2) is sodium hydroxideor potassium hydroxide.

It includes the alkali metal hydroxide aqueous solution obtained in Step(1) as alkali liquor and recycled to Step (1) to decompose the chromiteore.

The said aqueous solution in Step (4) includes sodium hydroxide orpotassium hydroxide aqueous solution with the concentration range from 0to 20% by weight.

The said acidifying agent in Step (5) includes inorganic acid, acidicgas, chromium anhydride, sodium dichromate, potassium dichromate, sodiumbicarbonate, potassium bicarbonate, sodium bisulfate, or potassiumbisulfate. The said inorganic acid includes sulfuric acid, hydrochloricacid, or nitric acid. The said acidic gas includes carbon dioxide orsulfur dioxide.

It also includes the aqueous solution obtained as the mothercrystallization liquor in Step (6) and recycled to Step (4) to dissolvethe primary chromate product.

Compared with the current-used methods for producing chromates, thepresent method has the following advantages:

(1) The reaction temperature in the present method is in the range from200 to 550° C., approximately 700° C. lower than the temperature of thecurrent roasting method. As a result, the low energy consumption, mildreaction conditions and strong industrial operability can be achieved.

(2) The chromium content of the ferrous residue in the present method isdecreased to below 0.5% by weight, and the recovery rate of chromiumapproaches 100%, 10 to 20% higher than that of the current industrialmethod. Therefore the resource utilization efficiency of the chromiumore is increased.

(3) Pure chromate crystal can be manufactured by purifying the primarychromate product obtained in the present method. Both the primarychromate product and the pure chromate crystal can be used as the rawmaterials to manufacture other chromium compounds.

(4) No auxiliaries are added in the present method and the amount ofdischarged residues is reduced from the source. The amount of theferrous residue produced is only 0.6 tonnes with the production of 1.0tonne of products, only a quarter of the amount of discharged residue inthe soda-ash roasting method. Furthermore, no dust or waste gas isproduced. The environmental pollution is therefore significantlyreduced.

(5) Almost no chromium is contained in the ferrous residue obtained inthis invention. The ferrous residue is an evenly mixed powder enrichedonly with iron and magnesium. It can be used as the iron-series rawmaterials in steel or cement industry after the extraction of magnesium.The disposal problem of the ferrous residue is resolved and theenvironmental pollution of the ferrous residue is eliminated as well.

(6) The alkali metal hydroxide used in this invention to decomposechromite ore can be separated and recycled, greatly reducing thechemical consumption of the alkali raw materials and significantlydecreasing the cost of raw materials.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates the technical flow sheet of this invention.

DETAILED DESCRIPTION OF THE INVENTION Example 1

Chromite ore with a particle size below 200 meshes, potassium hydroxide,and aqueous potassium hydroxide solution recycled to the reactor afterremoving part of the water are added in the reactor. The total amount ofpotassium hydroxide is 4 times by weight as much as that of the chromiteore. Air is then introduced to the reaction system. After completelymixed, the system is heated to 300° C. and remained for 10 hours. Amixture of alkali liquor, potassium chromate, and ferrous residue isobtained. The chromium conversion rate is higher than 99%.

Next, the mixture is leached with aqueous potassium hydroxide solutionwith a concentration of 30% by weight. The alkali concentration of theleached slurry is approximately 60% by weight. The primary potassiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.055% by weight after washing and drying.

After that, the primary potassium chromate product is dissolved inaqueous potassium hydroxide solution with a concentration of 1% byweight. Chromium anhydride is then added into the potassium chromateaqueous solution to adjust the pH value to 3. Pure potassium chromatesolution is obtained after filtering and removing the precipitates. Andpotassium chromate crystal is thus obtained after evaporating of thepurified potassium chromate aqueous solution, crystallizing, filtering,and drying. The purity of the potassium chromate crystal is 99.9%, andis up to the requirement of eligible product.

Example 2

Chromite ore with a particle size below 200 meshes, sodium hydroxide,and aqueous sodium hydroxide solution recycled to the reactor afterremoving all the water are added in the reactor. The total amount ofsodium hydroxide is twice by weight as much as that of the chromite ore.Oxygen is then introduced to the reaction system. After completelymixed, the reaction system is heated to 500° C. and remained for 20hours. A mixture of alkali liquor, sodium chromate, and ferrous residueis obtained. The chromium conversion rate is higher than 99%.

Next, the mixture is leached with aqueous sodium hydroxide solution witha concentration of 30% by weight. The alkali concentration of theleached slurry is approximately 45% by weight. The primary sodiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.15% by weight after washing and drying.

After that, the primary sodium chromate product is dissolved in aqueoussodium hydroxide solution with a concentration of 1% by weight. Carbondioxide is then introduced into the sodium chromate aqueous solution toadjust the pH value to 7. Pure sodium chromate solution is obtainedafter filtering and removing the precipitates. And sodium chromatecrystal is thus obtained after evaporating of the purified sodiumchromate aqueous solution, crystallizing, filtering, and drying. Thepurity of the sodium chromate crystal is 99.8%, and is up to therequirement of eligible product.

Example 3

Chromite ore with a particle size below 200 meshes, potassium hydroxide,and aqueous potassium hydroxide solution recycled to the reactor afterremoving part of the water are added in the reactor. The total amount ofpotassium hydroxide is twice by weight as much as that of the chromiteore. Oxygen is then introduced to the reaction system. After completelymixed, the reaction system is heated to 350° C. and remained for 6hours. A mixture of alkali liquor, potassium chromate, and ferrousresidue is obtained. The chromium conversion rate is higher than 99%.

Next, the mixture is leached with aqueous potassium hydroxide solutionwith a concentration of 20% by weight. The alkali concentration of theleached slurry is approximately 40% by weight. The primary potassiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.2% by weight after washing and drying.

After that, the primary potassium chromate product is dissolved inaqueous potassium hydroxide solution with a concentration of 5% byweight. Potassium bisulfate is then added into the potassium chromateaqueous solution to adjust the pH value to 8. Pure potassium chromatesolution is obtained after filtering and removing the precipitates. Andpotassium chromate crystal is thus obtained after evaporating of thepurified potassium chromate aqueous solution, crystallizing, filtering,and drying. The purity of the potassium chromate crystal is 99.8%, andis up to the requirement of eligible product.

Example 4

Chromite ore with a particle size below 200 meshes, potassium hydroxide,and aqueous potassium hydroxide solution recycled to the reactor afterremoving all the water are added in the reactor. The total amount ofpotassium hydroxide is 3 times by weight as much as that of the chromiteore. A mixture of oxygen and air is then introduced to the reactionsystem. After completely mixed, the reaction system is heated to 400° C.and remained for 4 hours. A mixture of alkali liquor, potassiumchromate, and ferrous residue is obtained. The chromium conversion rateis higher than 99%.

Next, the mixture is leached with water. The alkali concentration of theleached slurry is approximately 45% by weight. The primary potassiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.1% by weight after washing and drying.

After that, the primary potassium chromate product is dissolved inaqueous potassium hydroxide solution with a concentration of 20% byweight. Sulfuric acid is then added into the potassium chromate aqueoussolution to adjust the pH value to 4. Pure potassium chromate solutionis obtained after filtering and removing the precipitates. And potassiumchromate crystal is thus obtained after evaporating of the purifiedpotassium chromate aqueous solution, crystallizing, filtering, anddrying. The purity of the potassium chromate crystal is 99.7%, and is upto the requirement of eligible product.

Example 5

Chromite ore with a particle size below 200 meshes, sodium peroxide withthe same amount in mole as that of the chromite ore, sodium hydroxide,and aqueous sodium hydroxide solution recycled to the reactor afterremoving part of the water are added in the reactor. The total amount ofsodium hydroxide is 8 times by weight as much as that of the chromiteore. After completely mixed, the reaction system is heated to 300° C.and remained for 20 hours. A mixture of alkali liquor, sodium chromate,and ferrous residue is obtained.

Next, the mixture is leached with aqueous sodium hydroxide solution witha concentration of 25% by weight. The alkali concentration of theleached slurry is approximately 35% by weight. The primary sodiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.3% by weight after washing and drying.

After that, the primary sodium chromate product is dissolved in aqueoussodium hydroxide solution with a concentration of 5% by weight. Chromiumanhydride is then added into the sodium chromate aqueous solution toadjust the pH value to 4. Pure sodium chromate solution is obtainedafter filtering and removing the precipitates. And sodium chromatecrystal is thus obtained after evaporating of the purified sodiumchromate aqueous solution, crystallizing, filtering, and drying. Thepurity of the sodium chromate crystal is 99.9%, and is up to therequirement of eligible product.

Example 6

Chromite ore with a particle size below 200 meshes, potassium hydroxide,and aqueous potassium hydroxide solution recycled to the reactor afterremoving all the water are added in the reactor. The total amount ofpotassium hydroxide is 7 times by weight as much as that of the chromiteore. Air is then introduced to the reaction system. After completelymixed, the reaction system is heated to 450° C. and remained for 2hours. A mixture of alkali liquor, potassium chromate, and ferrousresidue is obtained. The chromium conversion rate is higher than 99%.

Next, the mixture is leached with aqueous potassium hydroxide solutionwith a concentration of 10% by weight. The alkali concentration of theleached slurry is approximately 35% by weight. The primary potassiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.05% by weight after washing and drying.

After that, the primary potassium chromate product is dissolved inaqueous potassium hydroxide solution with a concentration of 15% byweight. Nitric acid is then added into the potassium chromate aqueoussolution to adjust the pH value to 5. Pure potassium chromate solutionis obtained after filtering and removing the precipitates. And potassiumchromate crystal is thus obtained after evaporating of the purifiedpotassium chromate aqueous solution, crystallizing, filtering, anddrying. The purity of the potassium chromate crystal is 99.7%, and is upto the requirement of eligible product.

Example 7

Chromite ore with a particle size below 200 meshes, potassium nitratewith the same amount in mole as that of the chromite ore, potassiumhydroxide, and aqueous potassium hydroxide solution recycled to thereactor after removing part of the water are added in the reactor. Thetotal amount of potassium hydroxide is 6 times by weight as much as thatof the chromite ore. After completely mixed, the reaction system isheated to 250° C. and remained for 15 hours. A mixture of alkali liquor,potassium chromate, and ferrous residue is obtained.

Next, the mixture is leached with washing liquor of ferrous residue inwhich the potassium hydroxide concentration is approximately 5% byweight. The alkali concentration of the leached slurry is approximately70% by weight. The primary potassium chromate product, ferrous residue,and alkali liquor are then separated after filtering the leached slurry.The chromium content of the ferrous residue is 0.4% by weight afterwashing and drying.

After that, the primary potassium chromate product is dissolved inaqueous potassium hydroxide solution with a concentration of 10% byweight. Potassium dichromate is then added into the potassium chromateaqueous solution to adjust the pH value to 6. Pure potassium chromatesolution is obtained after filtering and removing the precipitates. Andpotassium chromate crystal is thus obtained after evaporating of thepurified potassium chromate aqueous solution, crystallizing, filtering,and drying. The purity of the potassium chromate crystal is 99.8%, andis up to the requirement of eligible product.

Example 8

Chromite ore with a particle size below 200 meshes, sodium hydroxide,and aqueous sodium hydroxide solution recycled to the reactor afterremoving all the water are added in the reactor. The total amount ofsodium hydroxide is 5 times by weight as much as that of the chromiteore. Air is then introduced to the reaction system. After completelymixed, the reaction system is heated to 550° C. and remained for 1 hour.A mixture of alkali liquor, sodium chromate, and ferrous residue isobtained. The chromium conversion rate is higher than 99%.

Next, the mixture is leached with aqueous sodium hydroxide solution witha concentration of 15% by weight. The alkali concentration of theleached slurry is approximately 50% by weight. The primary sodiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.1% by weight after washing and drying.

After that, the primary sodium chromate product is dissolved in water.Sodium bicarbonate is then added into the sodium chromate aqueoussolution to adjust the pH value to 8. Pure sodium chromate solution isobtained after filtering and removing the precipitates. And sodiumchromate crystal is thus obtained after evaporating of the purifiedsodium chromate aqueous solution, crystallizing, filtering, and drying.The purity of the sodium chromate crystal is 99.8%, and is up to therequirement of eligible product.

Example 9

Chromite ore with a particle size below 200 meshes, sodium nitrate withthe same amount in mole as that of the chromite ore, sodium hydroxide,and aqueous sodium hydroxide solution recycled to the reactor afterremoving all the water are added in the reactor. The total amount ofsodium hydroxide is 5 times by weight as much as that of the chromiteore. After completely mixed, the reaction system is heated to 400° C.and remained for 8 hours. A mixture of alkali liquor, sodium chromate,and ferrous residue is obtained.

Next, the mixture is leached with aqueous sodium hydroxide solution witha concentration of 10% by weight. The alkali concentration of theleached slurry is approximately 40% by weight. The primary sodiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.2% by weight after washing and drying.

After that, the primary sodium chromate product is dissolved in aqueoussodium hydroxide solution with a concentration of 10% by weight. Sodiumdichromate is then added into the sodium chromate aqueous solution toadjust the pH value to 6. Pure sodium chromate solution is obtainedafter filtering and removing the precipitates. And sodium chromatecrystal is thus obtained after evaporating of the purified sodiumchromate aqueous solution, crystallizing, filtering, and drying. Thepurity of the sodium chromate crystal is 99.8%, and is up to therequirement of eligible product.

Example 10

Chromite ore with a particle size below 200 meshes, potassium hydroxide,and aqueous potassium hydroxide solution recycled to the reactor afterremoving all the water are added in the reactor. The total amount ofpotassium hydroxide is 5 times by weight as much as that of the chromiteore. Oxygen is then introduced to the reaction system. After completelymixed, the reaction system is heated to 500° C. and remained for 1 hour.A mixture of alkali liquor, potassium chromate, and ferrous residue isobtained. The chromium conversion rate is higher than 99%.

Next, the mixture is leached with aqueous potassium hydroxide solutionwith a concentration of 15% by weight. The alkali concentration of theleached slurry is approximately 30% by weight. The primary potassiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.04% by weight after washing and drying.

After that, the primary potassium chromate product is dissolved inwater. Sulfur dioxide is then introduced into the potassium chromateaqueous solution to adjust the pH value to 9. Pure potassium chromatesolution is obtained after filtering and removing the precipitates. Andpotassium chromate crystal is thus obtained after evaporating of thepurified potassium chromate aqueous solution, crystallizing, filtering,and drying. The purity of the potassium chromate crystal is 99.7%, andis up to the requirement of eligible product.

Example 11

Chromite ore with a particle size below 200 meshes, potassium hydroxide,and aqueous potassium hydroxide solution recycled to the reactor afterremoving all the water are added in the reactor. The total amount ofpotassium hydroxide is 4 times by weight as much as that of the chromiteore. Air is then introduced to the reaction system. After completelymixed, the reaction system is heated to 550° C. and remained for 3hours. A mixture of alkali liquor, potassium chromate, and ferrousresidue is obtained. The chromium conversion rate is higher than 99%.

Next, the mixture is leached with aqueous potassium hydroxide solutionwith a concentration of 25% by weight. The alkali concentration of theleached slurry is approximately 55% by weight. The primary potassiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.035% by weight after washing and drying.

After that, the primary potassium chromate product is dissolved inaqueous potassium hydroxide solution with a concentration of 5% byweight. Hydrochloride acid is then introduced into the potassiumchromate aqueous solution to adjust the pH value to 5. Pure potassiumchromate solution is obtained after filtering and removing theprecipitates. And potassium chromate crystal is thus obtained afterevaporating of the purified potassium chromate aqueous solution,crystallizing, filtering, and drying. The purity of the potassiumchromate crystal is 99.8%, and is up to the requirement of eligibleproduct.

Example 12

Chromite ore with a particle size below 200 meshes, potassium peroxidewith the same amount in mole as that of the chromite ore, potassiumhydroxide, and aqueous potassium hydroxide solution recycled to thereactor after removing part of the water are added in the reactor. Thetotal amount of potassium hydroxide is 8 times by weight as much as thatof the chromite ore. After completely mixed, the reaction system isheated to 200° C. and remained for 20 hours. A mixture of alkali liquor,potassium chromate, and ferrous residue is obtained.

Next, the mixture is leached with water. The alkali concentration of theleached slurry is approximately 50% by weight. The primary potassiumchromate product, ferrous residue, and alkali liquor are then separatedafter filtering the leached slurry. The chromium content of the ferrousresidue is 0.5% by weight after washing and drying. After that, theprimary potassium chromate product is dissolved in water. Carbon dioxideis then introduced into the potassium chromate aqueous solution toadjust the pH value to 7. Pure potassium chromate solution is obtainedafter filtering and removing the precipitates. And potassium chromatecrystal is thus obtained after evaporating of the purified potassiumchromate aqueous solution, crystallizing, filtering, and drying. Thepurity of the potassium chromate crystal is 99.8%, and is up to therequirement of eligible product.

1. A method for producing chromates characterized in that it includes:the chromite ore is decomposed with oxidant in the molten salt or inaqueous solution of alkali metal hydroxide, the leaching slurry isobtained via leaching of the reaction products with aqueous solution,and the primary chromate products are separated from the leachingslurry; the method comprises following steps: (1) Chromite oredecomposing. A mixture of alkali metal hydroxide, alkali metal chromate,and ferrous residue can be obtained after the reaction of chromite orewith oxidant in the reactor in the molten salt or in aqueous solution ofalkali metal hydroxide; the weight of the alkali metal hydroxide used istwo to eight times as much as that of the chromite ore used; the amountof the oxidant is either equal to or above the stoichiometric amount fordecomposing chromite ore; the water amount in the reaction system is inthe range from 0 to 50% by weight of the total amount of all reactants;the reaction temperature is in the range from 200 to 500° C.; thereaction time is in the range from 1 to 20 hours; (2) To obtain a kindof slurry with an alkali metal hydroxide content of 30% to 70% by weightby leaching the mixture obtained in Step (1) with an aqueous solution ofalkali metal hydroxide with a concentration range from 0 to 30% byweight; (3) To separate the leaching slurry obtained in Step (2) andobtain respectively primary alkali metal chromate product, ferrousresidue, and alkali liquor.
 2. A method for producing chromates asclaimed in claim 1, wherein it also includes the purification of theprimary chromate products to manufacture pure chromate crystal. It iscarried out according to the following steps: (4) To dissolve theprimary chromate products obtained in Step (3) in aqueous solution andobtain chromate aqueous solution; (5) To adjust the pH value of thechromate aqueous solution obtained in Step (4) to the range from 3 to 9with acidifying agent, filter out the precipitates, and obtain purechromate aqueous solution; (6) To heat the pure chromate aqueoussolution to evaporate the water and obtain pure chromate crystal andmother liquor after crystallizing, filtering, and drying.
 3. A methodfor producing chromates as claimed in claim 1, wherein the said oxidantin Step (1) includes air, oxygen, sodium nitrate, potassium nitrite,sodium peroxide, potassium peroxide, or mixtures thereof.
 4. A methodfor producing chromates as claimed in claim 1, wherein the said alkalimetal hydroxide in Step (1) includes the alkali metal hydroxide recycledfrom Step (3), the alkali metal hydroxide supplemented, or mixturesthereof.
 5. A method for producing chromates as claimed in claim 1,wherein the said alkali metal hydroxide in Steps (1) and (2) is sodiumhydroxide or potassium hydroxide.
 6. A method for producing chromates asclaimed in claim 1, wherein it includes the alkali metal hydroxideaqueous solution obtained in Step (1) as alkali liquor and recycled toStep (1) to decompose the chromite ore.
 7. A method for producingchromates as claimed in claim 2, wherein the said aqueous solution inStep (4) includes sodium hydroxide or potassium hydroxide aqueoussolution with the concentration range from 0 to 20% by weight.
 8. Amethod for producing chromates as claimed in claim 2, wherein the saidacidifying agent in Step (5) includes inorganic acid, acidic gas,chromium anhydride, sodium dichromate, potassium dichromate, sodiumbicarbonate, potassium bicarbonate, sodium bisulfate, or potassiumbisulfate; the said inorganic acid includes sulfuric acid, hydrochloricacid, or nitric acid; the said acidic gas includes carbon dioxide orsulfur dioxide.
 9. A method for producing chromates as claimed in claim2, wherein it also includes the aqueous solution obtained as the mothercrystallization liquor in Step (6) and recycled to Step (4) to dissolvethe primary chromate product.